Method of refining metal



United States Patent fiice 2,920,362 Patented Jan. 12, 1960 METHOD OFREFINING METAL Davidlee Von Ludwig, Brooklyn, N.Y., assignor to SelasCorporation of America, Philadelphia, Pa., a corporation of PennsylvaniaNo Drawing. Application May '22, 1953 Serial No. 356,871

1 Claim. 01. 22-216) The present invention relates to the refining anddensifying of metals, and more particularly to the use of vibrationduring the refining period. The present application is -a continuationin part of my copending application, Serial No. 61,132, filed November19, 1948, now abandoned.

Prior efforts to refine metals in vacuum have not been commerciallysuccessful for several reasons. The time required to obtain anappreciable freedom from gaseous impurities in the metal when the cycleof outgassing normally attempted in vacuo has been used has beenexcessive, and the degree of purification obtainable in such treatmentshas not been as great as desired. The best prior technique employed forvacuum refining metals has been incapable 'of reducing the quantity ofnongaseous impurities, such as slags, stable oxides or nitrides or othermetalloids contained in the melt. These deficiencies have been the basisfor beliefsthat no benefits of practical value can acrue from theproductionof castings in vacuum. While 'it is desirable to removenonmetallic contaminants from ingots to be subsequently remelted andformed into castings, it is even more important to produce ingots freefrom discrete solid impurities when such ingots are to be forged orotherwise mechanically worked. In such cases minute impurities thatmight not produce a fault in a cast structure, when elongated by forgingor other working, frequently become sources of weakness on preferredlines of metal breakdown. In most cases fatigue failure planes are offar greater proportional magnitude to the reliability of forgedstructures than to cast structures.

It is an object of the present inventionto provide a method of removinggases and slags or other undesirable stable contaminants from metal. Itis a further object to provide a method of refining or purifying metalsor alloys more rapidly and to a higher degree than has hitherto beenpossible. V

In following my invention the metal to be refined may be melted in ahigh vacuum. While in its molten state the metal is subjected tovibration of controlled frequency and amplitude so that the melt isthoroughly agitated. Such vibrations produce a segregation of the'metaland the contaminants in accordance with their relative specificgravities. The vibration also speeds up appreciably the precipitationand expulsion of any gas that may be in the metal, and materiallyincreases the eifectiveness of gas removal. 7

It is a further object of my invention to use controlled vibrations toprecipitate and segregate undesired concontrolled and the segregation ofalloying elements in the ingot mass controlled.

The various features of novelty which characterize my invention arepointed out in the claim annexed to and forming a part of thisspecification. For a better understanding of the invention, however, itsadvantages, and specific objects attained with its use, reference shouldbe had to the description set forth in detail below.

In accordance with the principle of this invention, if the treatment isto take place in an environment of sub- 'atmospheric pressure,relatively large masses of metal, either pure or of some particularalloy, are placed in a melting crucible and the crucible put in anapparatus and tightly sealed. The pressure in the apparatus is thendecreased until a suitable value thereof has been reached. This value iseffectively between one and ten microns mercury pressure, except whenthe melt contains metals which sublime, boil, or otherwise are lost fromthe melt at such pressures. In this case the degree of evacuation usedin the refining chamber will be as low as possible but high enough toavoid excessive metal loss. During the time that the pressure change istaking place, or after the proper pressure level has been attained, thecrucible and charge will be heated to melt the metal by electricresistance or induction coils or by suitably muffled gas or oil burners.The temperature of the melt will be maintained sufiiciently above themelting point to insure complete liquidity in order to permit readyescape of gases, a uniform temperature being held until pressureequilibrium at the desired refinement level' in the system isre-attained. In this invention the molten metal is to be subjected tovibration from the moment liquefication is complete in order to expeditethe expulsion of gases, throughout the period of holding at constanttemperature while stabilizing the system pressure. While the melt isbeing vibrated, after the pressure has been stabilized at the highesttemperature to which the metal is subjected, the heat input is stoppedand the melt is cooled until solidification has commenced, while thevibration is continued. Cooling is expedited by passage of water throughthe induction coils, or by passages provided in the apparatus for thatpurpose. The metal is then remelted preparatory ;to discharge from thecrucible into ingot molds of suitable sizes and shapes. If desired, themetal can be poured without freezing and remelting. While the metal isbeing poured into the ingot molds, the molds. are caused to vibrate,preferably in a vertical plane, for the purpose of separating the stablenon-gaseous slags or other impurities which could not be removed in theprior phases of the refining treatment. After solidification of theingots is complete, the pressure in the apparatus may be returned'toatmospheric. It is generally desirable to tra-nsfor the hot, refinedingots to a cooling chamber in which a suitable controlled atmospherecan be maintained in contact with the cooling metal to minimize thedepth of oxide or nitride or other contamination of the ingot surface.

Almost total exclusion of gas and solid contaminants can be obtainedduring the refining cycle mentioned above if the metal is vibrated atthe correct frequency and in the proper direction during the cooling andslow solidification phase of the refining cycle. The vibration can andshould also take place during the cooling of the metal in the ingotmolds until the ingots are completely frozen.

In casting ingots of refined and degassed metal for sub sequentremelting and casting into desired shapes, the chief characteristicrequired is that of the exclusion of slags or other non-metallic stablecontaminants. If no particular effort is made to avoid segregation orerratic grain growth, the vibration frequency and amplitude can be thesame for most alloys. The grain growth and alloy segregation areautomatically corrected when the ingots are remelted for casting to thedesired shapes. The term vibration as used herein should not be confusedwith ordinary shaking or stirring of the metal.

The vibration producing effective exclusion of gases, slags and stablemetalloids from metals and alloys will vary with the composition of thematerial being treated. It is necessary to determine the properfrequency range of the vibrations as well as the range of amplitude andplanes of orientation. My experiments indicate that each group of alloysbased upon any particular metal or metal group has a sufficiently broadrange of response to permit of practical controls being established foralloy groups rather than for each individual alloy. The above notedrange of the vibrations for any particular metal or family of alloysmust be determined empirically. The proper amplitudes and frequencieswhich produce improved cleanliness of metals when applied to them in avacuum are the same as those determined upon the metal in normal airpressures. Therefore, the experimental deter mination of the propervibrations for any given composition may be determined withoutdifliculty outside the vacuum environment. Generally these vibrationsthat speed up degassing and produce segregation of impurities will havefrequencies in the range of from 500 to 5000 cycles per minute with anamplitude within the range of from to The vibration preferably isdirected in a vertical plane and has the form of a sine wave. The ingotsso produced will be found to have most of the impurities in the top orbottom layer of metal, depending upon the relative specific gravities ofthe impurities and the metal containing them. Once isolated in thesespecific ingot sections, it becomes a simple matter to grind or machineaway the layers containing the impurities.

The proper vibrations to be applied during the several phases of therefining cycle are determined in accordance with the function they areintended to perform. Most effective exclusion of gases is obtained byvibration during the period of cooling and slow, controlledsolidification in the crucible during which the entire melt is vibratedprior to being reheated for casting into ingots. For the purpose ofsegregating non-gaseous contaminants, the ingot molds are vibratedduring the time of solidification of the ingot metal, the vibration tohave commenced as soon as the metal begins to fill the ingot molds. Thevibration may be accomplished by a stirring mechanism which is equippedwith a vibrating device the stirring mechanism would then impart thevibrations to the molten metal. In some cases, for the reasons ofchemistry or temperature or both, no suitable material for the stirringdevice may be available. The metal may then be caused to vibrate bymeans of a device fastened to the crucible or the ingot molds or thesupports therefore. Preferably the vibrating device, of what ever kindused, will be self contained and electrically operated, so that nomoving connections are required to extend through the walls of a vacuumtight container when the refining is taking place in such a container.

When the metal being refined is to be used for forging or to beotherwise cold or hot worked without being reliquefied, the vibrationtreatment must be accurately regulated while the metal is being castinto the ingot molds and while it is solidifying therein. Whereas thelow frequency vibration of the ingot molds in the vertical plane will inall cases separate the non-gaseous contaminants, it will also tend tocause the various alloying elements not in true solution in the basemetal to be undesirably separated. Both alloy segregation and grain sizemay be controlled by the use of a high frequency vibration of lowamplitude applied in any convenient direction and of such an order ofmagnitude, with respect to the low frequency vibration beingsimultaneously applied, as to avoid the establishment of nodes ofinterference or amplification of the several wave patterns. Suchvibration may be imparted to the ingot mold by any well known highfrequency vibrating device in which the vibrations produced can becontrolled as to frequency and amplitude. My work ,has demonstrated thatit is possible to attain a controlled grain size of almost any desiredmagnitude by the correct use of the high frequency, low amplitudevibration in conjunction with the low frequency vibration and that, allother factors being equal, the grain size in the solidified ingotbecomes increasingly smaller as the frequency of vibration rises.

When a high frequency, low amplitude vibration is used, the valuesthereof must be determined empirically in the manner described above.This vibration will generally be from 2500 cycles per minute to 35000cycles per minute, but will in all cases be at least ten times as fastas low frequency vibration and never an exact multiple thereof. Theamplitude will be in the range of from 0.0005" to 0.005" thoughgenerally speaking 0.00 has been adequate and no apparent gain isattained by an excessive amplitude of vibration at the higher frequency.The Wave form of the low frequency vibration will take the form of asine curve when producing ingots which are to be forged, in order toavoid inter-grannular cracking and other mechanical weakening of theingot structure. Whether one or two vibrations are used, the ingotshould be vibrated during solidification until all metal phases aresolid. The entire ingot mass solidifies at a higher temperature levelthan normally is attained in the solidification of metal without the useof vibration. This fact further reduces the time of the vacuum refiningtreatment.

While the use of vibration in refining and degassing metal isadvantageous in any enviroment, it is particularly advantageous whenused in a refining operation taking place in vacuum.

As a specific example of a treatment of one alloy and the improvedresults that are obtained, the following is given.

In this example, the metal treated is an alloy of aluminum consisting of5% silicon and aluminum. The alloy is placed in a crucible which issurrounded by a container that is evacuated. Evacuation and heating ofthe alloy are started and the rate of evacuation is such that a pressurewhich is equivalent to one micron of mercury is obtained by the time thealloy has reached a pasty condition indicating that it is at a point oftransition from a solid to a liquid. The pressure of the container inwhich the melting takes place is at about one micron of mercury or at apartial pressure that is equivalent thereto, which can be about 50microns of mercury with an atmosphere neutral to the metal such ashelium or argon. The purge pressure of about 50 microns of inert gas isphysically and chemically the same as a total vacuum in as far as theequilibrium of dissolved gases in the metal is concerned and can beobtained in considerably less time and with less equipment than isrequired for a higher vacuum. During the time that the metal is melted,and beginning at the time that it starts to change from solid to liquid,vibration is ap plied to the crucible in which the melting is takingplace. This vibration is in any direction at a frequency of 5,000 cyclesper minute and an amplitude of 0.05". The vibration, along with thereduced atmosphere causes the dissolved gases in the alloy to beexpelled or precipitated.

After the metal is liquefied, it is held at its liquid temperature ofbetween 1225 F. to 1300 F. while being vibrated until such time as thepressure in the container reaches equilibrium, indicating that thedissolved gas in the metal has been removed for the temperatureequilibrium of the liquid metal. The metal is then solidified by coolingit in any desired manner, and the vibration must be maintained until themelt has solidified. The entire time for degassing will take from 30 to45 minutes as compared to over 10 hours at the same pressure if novibration is used. The reduced atmospheric pressure around the crucibleshould be maintained, however.

The metal is then remelted and discharged into ingot molds. For thispurpose, it is desirable to use a bottom discharge type of crucible,since the casting can be easier accomplished in a container having areduced atmosphere with such a crucible than if some mechanism isrequired to tilt the crucible for pouring. If the ingots are to beremelted, the form of the grain structure produced is more or lessimmaterial since any grain structure that is obtained would be lost uponremelting for casting. During the casting, however, the ingot mold isvibrated at a frequency of about 2500 cycles per minute and theamplitude ranging from A to of an inch. These vibrations are applied ina vertical direction and have the form of an undamped sine wave. Thepurpose of this vibration and its practical effect is to separatevarious insoluble contaminants and slag which will segregate to eitherthe top or the bottom of the ingot depending upon the relative specificgravity of the alloy and the contaminants. In this aluminum alloy, thedrosses are forced to the top. Upon solidification and cooling of theingots, the contaminants can then be removed by grinding or machiningthem from the end of the ingot.

If the ingots are to be used directly for forging or rolling or otherpurposes, the vibration of 2500 cycles per minute with an amplitude of Ato 7 of an inch is applied as in the above example, There is alsoapplied a second vibration of a frequency of approximately 26000 cyclesper minute, or at least ten times the low frequency vibration, and anamplitude of 0.005". This vibration is also an undamped sine wave but itcan be applied in any direction as long as it is applied to the mold andhas an effect upon the molten metal. The low frequency, high amplitudevibration in this case also serves to segregate the impurities from themetal of the alloy. The high frequency and low amplitude vibrationserves to prevent segregation of the alloy constituents and to produce afine, dense grain in the alloy, eliminating center shrinkage or pipe.

In either case, whether the ingot is to be remelted or is to be used forrolling or forging the vibrations are continued until solidification ofthe metal is completed.

The above treatment will produce metal with a tensile strength in theneighborhood of 27,000 pounds per square inch with an elongation of from10% to 30% coupled with extremely fine, dense grain structure. Thisparticular alloy with ordinary casting will have a tensile strength ofabout 17,000 pounds per square inch with from 3% to 6% elongation. Itwill, therefore, be seen that the physical qualities of the alloy aregreatly improved by the application of the vibration during the refiningprocess.

The method of treating metal described above is equally as effective onother metals such as copper and ferrous alloys, for example, when thefrequencies and amplitudes of the vibrations have been determined in themanner described above.

The above specific example of aluminum alloy was described ducedatmosphere. The same treatment can, of course, take place in the air atatmospheric pressure. In such a case, however, there is some loss ofmaximum outgassing. Maximum segregation of impurities and refinement ofthe treatment of a given as taking place in a re the high frequency,

the metal grain structure can be obtained by the use of low amplitudeand low frequency, high amplitude vibrations regardless of theenvironment in which they are applied.

The application of vibrations to metal during refining not onlydecreases the time required for treatment at any given temperaturepressure level to produce thorough outgassing of the metal, but also,when used in the manner outlined above, effectively segregates thenon-gaseous contaminants such as slags, stable oxides, nitrides,sulphides and similar metalloids, concentrating these undesirableelements in specific layers of the ingot metal, either at the top orbottom of the metal mass. Use of vibrations in the manners outlinedabove not only accelerates the tendency for gases to separate themselvesfrom the metal, but also overcomes the tendency for the precipitatedgases to be held as discrete voids within the solidifying metal,wherefrom they may be readily re-dis solved upon subsequent remelting ofthe ingot, or wherein they may become the source of mechanical flaws inthe body of the object which has been formed from the ingot metalcontaining trapped gas bubbles.

While in accordance with the provisions of the statutes, I havedescribed the best embodiment of my invention now known to me, it willbe apparent to those skilled in the art that changes may be made withoutdeparting from the spirit and scope of the invention as set forth in theappended claim, and that in some cases certain features of the inventionmay be used to advantage without a corresponding use of other features.

What is claimed is:

The method of processing metal to remove gas andnon-metalliccontaminants which comprises placing the metal in an enclosed space,reducing the pressure in said space to an effective pressure below 50microns of mercury, applying heat to the metal to melt the same andreaching the minimum pressure by the time the metal has been heated to atemperature at which it is beginning to liquefy, vibrating the metal ata frequency of from 500 to 5000 cycles per minute at an amplitude offrom /6 to 2 from the time it begins to liquefy, after the pressure inthe space has reached equilibrium pouring the metal into a mold,vibrating the mold in a vertical direction at a frequency of from 500 to5000 cycles per minute at an amplitude of from ,4 to from the timepouring is started until solidification in the mold is complete, andreturning the pressure in the space to atmospheric.

References Cited in the file of this patent UNITED STATES PATENTS1,318,740 Fessenden Oct. 14, 1919 1,956,910 Roth May 1, 1934 2,660,414Non Ludwig Nov. 24, 1953 FOREIGN PATENTS 138,648 Great Britain May 5,1921 683,996 France Mar. 10, 1930 456,657 Great Britain Nov. 9, 1936479,933 Great Britain Feb. 8, 1938 480,554 Great Britain Feb. 24, 1938

